The processing of carbonaceous feedstocks to produce heat, chemicals or fuels can be accomplished by a number of thermochemical processes. Conventional thermochemical processes, such as combustion, gasification, liquefaction, and conventional pyrolysis are typically equilibrium processes and yield relatively low-value equilibrium products including major quantities of non-reactive solids (char, coke, etc.), secondary liquids (heavy tars, aqueous solutions, etc.), and non-condensible gases (CO.sub.2, CO, CH.sub.4, etc.). For example, combustion is restricted to immediate thermal applications, and gasification normally produces low energy fuel gas with limited uses. Liquefaction and conventional pyrolysis often produce low yields of valuable liquid or gaseous products. In addition the liquid products which are produced often require considerable secondary upgrading.
Pyrolysis is characterized by the thermal decomposition of materials in the relative absence of oxygen (i.e., significantly less oxygen than required for complete combustion). Typically, pyrolysis has historically referred only to slow conventional pyrolysis whose equilibrium products included roughly equal proportions of non-reactive solids (char and ash), secondly liquids, and non-condensible gases.
However, over the past two decades fundamental pyrolysis research has unexpectedly indicated that high yields of primary, non-equilibrium liquids and gases (including valuable chemicals, chemical intermediates, petrochemicals and fuels) could be obtained from carbonaceous feedstocks through fast (rapid or flash) pyrolysis at the expense of undesirable, slow pyrolysis products. In other words, the low-value product distribution of traditional slow pyrolysis can be avoided by the approach embodied by fast pyrolysis processes.
Fast pyrolysis is a generic term that encompasses various methods of rapidly imparting a relatively high temperature to feedstocks for a very short time, then rapidly reducing the temperature of the primary products before chemical equilibrium can occur. By this approach the complex structures of carbonaceous feedstocks are broken into reactive chemical fragments which are initially formed by depolymerization and volatilization reactions, but do not persist for any significant length of time. Thus, non-equilibrium products are preserved, and valuable, reactive chemicals, chemical intermediates, light primary organic liquids, specialty chemicals, petrochemicals, and/or high quality fuel gases can be selected and maximized at the expense of the low-value solids (char, coke, etc.), and heavy secondary organic liquids (tars, creosotes, etc.).
Fast pyrolysis is an intense, short duration process that can be carried out in a variety of reactor systems. The common aspect of these reactors is the ability to achieve extremely rapid feedstock heating with limitation of the reaction to relatively short times by rapid cooling which stops the chemical reactions before the valuable intermediates can degrade to non-reactive, low-value final products. A fast pyrolysis process reactor system must therefore be characterized by the following requirements:
1) A very rapid .feedstock heating rate. Typically the heating rate lies within the range of 1,000 to 1,000,000.degree. C. per second. PA1 2) A controlled, elevated reaction temperature. Typically the reaction temperature lies within the range of 350 to 800.degree. C. PA1 3) A controlled, short reaction/residence time. Typically the residence time lies within the range of 0.03 seconds to 2 seconds. PA1 4) A rapid product quench. Typically the products are quickly cooled below 350.degree. C within 0.5 seconds. PA1 1) Relatively high yields of the desirable products; PA1 2) Scalability of the process to industrial size reactors; PA1 3) Industrially practical operation (reasonable energy requirements, durability, process controllability, etc.). PA1 1) a very short, uniform, controlled residence time; PA1 2) extremely rapid, thorough mixing in the mixing section to ensure very high heat transfer rates; PA1 3) adequate heat supply and transfer to the reaction zone via only the circulating particulate solid heat carrier; PA1 4) avoidance of cyclone flooding while achieving minimum desirable reactor residence times; PA1 5) a very high degree of particle ablation; PA1 6) non-oxidative conditions in the reaction zone; PA1 7) effective separation of the condensible vapour products from the heat carrier solids without loss of the condensed vapours to the solids recirculation stream; PA1 8) cleaning and recycle of a portion of the non-condensible product gases for use as a transport medium; PA1 9) prevention of pre-pyrolysis in the feed system; PA1 10) prevention of plugging of the reactor at high loading ratios; PA1 11) a reactor configuration that permits use of a sufficiently high ration of inorganic particulate heat carrier to feedstock. PA1 1) Ablation of the reacting particles. That is, a physical/mechanical mechanism that removes the primary depolymerization liquids from the reacting surface at a surface regression rate that is consistent with the thermal penetration rate. In effect, at an infinitesimal distance below the retreating reaction surface, the temperature remains far below the reaction temperature and very limited fragmentation, depolymerization or repolymerization reactions occur. PA1 2) Minimal back mixing within the reactor. That is, there is very minimal internal recirculation (eddies) of the reactant, carrier gas, products or heat carrier solids within the reacting zone. PA1 3) Precise control of a uniform short reactor residence time. This implies that the average residence time not only be short but that there be very limited or no residence time distribution about this average. PA1 4) Control of the loading ratio of inorganic heat carrying particles to feedstock above 12:1. PA1 a) introducing a primary stream of carbonaceous material and a secondary stream of upwardly flowing inorganic particulate heat supplying material into the mixing section in the relative absence of oxygen, the ratio of the mass of inorganic heat supplying material: mass of carbonaceous feedstock greater than 12:1; PA1 b) maintaining the stream of carbonaceous material in contact with the secondary stream of heat supplying material through the reactor section to cause transformation of the carbonaceous material to a product stream; PA1 c) separating the product stream from the beat supplying material by separation means at the exit of the reactor section such that the average residence time of contact between the carbonaceous material and the heat supply material is less than 2.0 seconds and the temperature of the products is reduced after exiting from the reactor section to less than 300.degree. C. in less than 0.1 seconds; PA1 d) recycling the heat supplying material to the mixing section. PA1 a) a mixing section having a first inlet means for the introduction of heat carrying inorganic particulate material and secondary inlet means for the introduction of carbonaceous material; PA1 b) an upflow reactor section is situated above the mixing section; PA1 c) separation means at the outlet of the reactor section to separate the gaseous and liquid pyrolysis products from the heat carrying inorganic particulate solids; PA1 d) controlled gravity fed recirculation line between the separation means and mixing section, the controlled gravity fed recirculation line for returning the inorganic heat carrying particulate solids to the mixing section; and, PA1 e) condensing means for cooling and condensing liquid pyrolysis products after exiting the separation means. PA1 1) extremely rapid heat transfer from the solid heat carrier to the carbonaceous reactants such that the reactants reach the desired reaction temperature in a fraction of the overall desired residence time; PA1 2) precise control of a uniform, very short residence time such that maximum non-equilibrium yields of total liquids or selective maximum yields of individual chemicals, fuels or classes of chemicals are achieved; PA1 3) excellent particle ablation such that undesirable secondary reactions within the reacting particle are minimized to limit production of heavy secondary tars and solid residue products (char, coke, carbon fines); PA1 4) very limited back-mixing such that the residence time distribution is narrow and secondary reactions arc minimal; PA1 5) a controlled elevated temperature; PA1 6) a configuration amenable to rapid product quench; PA1 7) means for effective recycling of the hot solid heat carrier.
Furthermore, in a true fast pyrolysis system where liquids are the desired primary product, product data as characterized in Table I is typically obtained.
TABLE 1 ______________________________________ FAST AND SLOW PYROLYSIS CHARACTERIZATION PROPERTIES FAST* SLOW ______________________________________ MASS YIELDS (%): Oil/Tar 75-90 15-40 Aqueous Phase 0 10-15 Gas 7-11 20-40 Char 3-14 20-35 LIQUID PRODUCT: (Oil/Tar) Phases one two Viscosity (cp @ 40.degree. C.) 40-100 300 plus Moisture (%) 15-30 10 Energy Content (MJ/kg) 16-18 26 Pour Point (C.) -23 32 Acidity strongly weakly acidic acidic Blendable in No. 2 no unknown Fuel Oil ______________________________________ *References for the fast pyrolysis data represent several researchers using various fast pyrolysis techniques.
Accordingly, a true fast pyrolysis system, optimized for liquids production, is characterized by relatively high oil/tar yield, with no distinct aqueous phase yield, low gas yield and low char yield. Furthermore, a true fast pyrolysis system will yield a single liquid phase.
Past reactor systems have been limited to slow pyrolysis processes by a variety of factors including the selection of the heat carrier used to carry out the rapid heating of the process and by carrying out the process at too low loading ratios of particulate heat carrying material to feedstock. For example, past processes have relied upon organic heat carriers, such as hot char to provide the desired feedstock heating rate. The use of organic heat carriers results in primarily non-contact radiation and convection heat transfer as opposed to the more desirable ablative heat transfer and thereby results in a slow pyrolysis process.